At some point there are those who will want concrete policy answers from the guy….

He boasts about spending no time preparing for the event, which will be broadcast on Fox News on Aug. 6, even as his aides have put together briefing papers for him on policy and pungent lines of attack. He already knows plenty about the issues, he says, so much that, rather than cramming, he will be in Scotland over the weekend at a golf tournament on one of his courses.

And after weeks of slashing at his opponents in interviews, he refuses to say whom he may single out when the 10 leading primary contenders stand side by side. “I have great respect for some of the candidates,” Mr. Trump said in an interview. “I don’t have great respect for others.”

He cannot know who will try to embarrass him. Then again, he suggested, he may just choose targets of opportunity. “It depends on the feel,” he said. “It depends on what’s taking place.”

No candidate is more likely to wing it than the mercurial Mr. Trump. But the man who read Senator Lindsey Graham’s cellphone number aloud on a South Carolina stage has set the bar fairly high for himself to do something that would qualify as outrageous.

He is likely to arrive in Cleveland ready with cutting “observations” about each of his rivals, according to a person briefed on Mr. Trump’s debate preparations. (A recent example: He said former Gov. Rick Perry of Texas “put glasses on so people will think he’s smart.” He added, “It just doesn’t work.”)

Mr. Trump’s mantra, in his books and in his paid speeches, is to counterattack harder when anyone throws a punch……